Project Summary/Abstract Infections transmitted by ticks have increased in incidence worldwide as a result of environmental changes. Important aspects of the induced disease pathogenesis facilitate the identification of preventive and/or therapeutic alternatives to more severe illnesses. In ehrlichiosis and other vector borne diseases there is a lack of information regarding the initiation of infection, due to the difficulties associated with a model using the natural mode of infection. Our previously developed tick transmission model allowed us to identify important differences in mortality and bacterial distribution by using different routes of infection. However, limited knowledge is available about the site of infection and host response to pathogen-tick interaction. This proposed project will identify the immune pathways involved in early pathogenesis of tick borne ehrlichiosis. Our model demonstrates severe inflammation and antigen persistence in the skin area of tick inoculation of bacteria. Interestingly, different routes of infection induced different disease outcome and bacterial dissemination. Our preliminary results identify significant expression of the IL-36 group of cytokines during initial ehrlichial transmission at the site of tick feeding, which could potentially mediate inflammatory cells recruitment. Therefore, the objective of our research is to determine the role of IL-36 in initiation of infection during tick transmission of the ehrlichial pathogen. Our hypothesis is that activation of skin KCs by tick-transmitted ehrlichiae results in IL-36 mediated inflammation, which facilitates establishment of infection in recruited monocytes/macrophages (Mo/M?s). We will first demonstrate KCs as the primary source of the IL-36 family of cytokines during ehrlichiae-tick transmission, and further identify the role of IL-36 to drive inflammatory processes. Furthermore, we will demonstrate how the recruitment of monocytes/macrophages by these inflammatory mechanisms facilitates infection and dissemination during early transmission. By accomplishing the goals of this project we expect to demonstrate the importance of the early activation of IL-36 as a key mediator in the skin at site of ehrlichial infection as part of disease pathogenesis. Understanding the key determinants underlying the establishment of infection will enable us to identify markers of susceptibility to, and new therapeutic targets for, tick borne ehrlichiosis.